Its easy to do.
First make your hair material and apply it to the patches you want to have hair on.
Then apply the decal as you would a normal decal. I think it needs to cover all of the hair patches, so make sure that the edge of the decal covers all the patches that will have the hair material applied to them.
Once the decal is stamped, change its type under the image properties, from the default "color" to "Other..."
This will bring up an option called "properties driven" in the properties other the image type.
Right mouse click on the "properties driven" property and select... "Add Property". This will add a property to control one of four things with the hair. diffuse Color, Length, Density, and Direction. You'll need to add one property for each thing you want to control.
The Color should be a color map. The density and length should be grayscale maps (where Black is the low number.. ie no hair or very short, and white is the large number ie... full density and very long) The Direction map is tricky and I wrote an app to help with painting direction maps.
Basically direction maps are controlled with either the red or blue of the RGB of an image. The green allows for 256 tweeks to the 256 shades of either red or blue. You probably wouldnt need any green. I wrote a tutorial and a paint tool to paint these types of maps. It also talks about the application of the other types. Its at...http://www.colins-loft.net/hairbrush.html
The tutorial was written for the old hair system so the images show some things you won't see like Kinkiness maps, etc. But the process is still the same.
Hope that helps